Prostate cancer is a widespread medical problem. Histological evidence of prostate cancer occurs in an estimated 30-40% of men over the age of 50, yet only 9% of men develop clinical signs or symptoms of prostate cancer and 3% die from the disease each year. (Chirarodo, 1991) Early prognosis often can prolong the longevity of the patient. Diagnosis and monitoring of prostate cancer, however, has been problematic. One method that has been used is to measure the amount of prostate specific antigen (PSA) in the serum of patients, given that there appears to be some correlation between increased levels of PSA and the presence of prostate cancer. The benefits of such serum PSA measurements, however, are controversial because of the rather low specificity of this test. Lilja et at., Cancer Supplement 70(1):230-234 (1992). PSA has been shown to be expressed in normal prostate glands, as well as in cancerous prostate glands. Moreover, while elevated levels of PSA indicate an increased likelihood of the presence of prostate cancer, such elevated levels are also found in a high percentage of patients with benign prostatic hyperplasia and other benign urological disorders. Somewhat improved differentiation between prostate cancer and benign prostatic hyperplasia has been reported if the level of PSA-ACT (.alpha.1-antichymotrypsin) complex present in patient serum is measured rather than if PSA levels alone are measured. Stenman et al., Cancer Res. 51:222-261 (1991), Lilja et al., Clin. Chem. 37:1618-1625 (1991); Christensson et al., J. Urology 150:100-105 (1993). This procedure too, however, is not highly specific for prostate cancer. There is a need for an accurate diagnostic method that can distinguish between prostate cancer and benign prostatic hyperplasia.